1. Field of the Invention
The present invention relates to an image-shake correcting device suitable for use in a photographic apparatus such as a camera.
2. Description of the Related Art
In the field of photographic apparatus such as cameras, various photographic functions, such as exposure setting and focus adjustment, have heretofore been automated, and a greater number of functions have been incorporated into a single camera. Accordingly, photographers can enjoy photography at any time irrespective of photographic environments.
In spite of many innovations made in this art, there is the problem that the quality of a photographic image is often remarkably impaired by a camera shake during actual photography. In recent years, various image-shake correcting devices have been proposed and receiving much attention.
Although various forms of image-shake correcting devices are considered, they are generally classified into several types in terms of the kind of correcting system or detecting system used therein. One type of image-shake correcting device uses a correction system for optically correcting an image shake, and another type uses a correction system for electrically correcting an image shake by using image processing. Yet another type uses a detecting system for physically detecting a vibration, while a further type uses a detecting system for detecting a motion vector or the like of an image by image processing.
FIG. 1 is a block diagram showing one example of a proposed image-shake correcting device. Referring to FIG. 1, a gyro (angular-velocity sensor) 1 is mounted in the body of a photographic apparatus such as a camera, and is arranged to physically detect a vibration applied to the photographic apparatus, in the form of an angular velocity and output an angular-velocity signal. A DC-cut high-pass filter 2 (hereinafter referred to the "HPF") is provided for eliminating a direct-current component from the angular-velocity signal outputted from the angular-velocity sensor 1, thereby causing only a vibration component to pass through the HPF 2. An integrator 3 is provided for integrating the vibration component passing through the HPF 2, computing an average value of the vibration component, and outputting an angular-velocity signal. The angular-velocity signal serves as an evaluation value indicative of the vibration of the photographic apparatus.
A variable angle prism (hereinafter referred to as the "VAP") 9 includes two transparent parallel plates 91 and 92 which are opposed to each other, and an elastic substance or inactive liquid 93 made from a transparent material of high refractive index is hermetically enclosed in the space between the transparent parallel plates 91 and 92. The space between the transparent parallel plates 91 and 92 are elastically sealed around the external circumference thereof by a sealing material 94, such as a resin film, so that the transparent parallel plates 91 and 92 are relatively swingably arranged. By varying the relative angle made by the two transparent parallel plates 91 and 92 by means of a mechanical driving produced by an actuator 7, the apex angle of the VAP 9 is made to vary, thereby varying the angle of incidence of a light flux upon a lens unit 10. The state of driving of the VAP 9, i.e., the apex angle, is detected by an apex angle sensor 8 as a displacement angle relative to the position at which the two transparent parallel plates 91 and 92 are parallel to each other.
The arrangement shown in FIG. 1 also includes an adder 4 for performing an opposite-polarity addition (subtraction) of the output signal of the apex angle sensor 8 to the angular-displacement signal outputted from the integrator 3, an amplifier 5 for amplifying the output signal of the adder 4, and a driving circuit 6 for converting the output signal of the amplifier 5 into a driving signal to be applied to the actuator 7 for driving the VAP 9.
More specifically, in the adder 4, the angular-displacement signal obtained by causing the integrator 3 to average the vibration component detected by the angular-velocity sensor 1 is subtracted from the amount of variation of the apex angle of the VAP 9 which is outputted from the apex angle sensor 8, thus preparing the difference therebetween. The amplifier 5 and the driving circuit 6 control the actuator 7 to drive the VAP 9 in the direction in which the difference is made "0". The resultant displacement of the apex angle of the VAP 9 is detected by the apex angle sensor 8 and supplied to the adder 4.
Accordingly, a closed loop is formed which starts with the adder 4, passes through the amplifier 5, the driving circuit 6, the actuator 7, the VAP 9 and the apex angle sensor 8, and returns to the adder 4. The VAP 9 is controlled so that the output signal of the adder 4 is made "0", i.e., the angular-displacement signal supplied from the integrator 3 and the signal indicative of the apex angle, outputted from the apex angle sensor 8, coincide with each other at all times. Thus, image-shake correction can be effected.
The light flux the angle of incidence of which has been changed by the VAP 9 is focused on the image pickup surface of an image pickup device 11, such as a CCD, by the lens unit 10, and an image pickup signal obtained by photoelectrically converting the incident light flux is outputted from the image pickup device 11.
The aforesaid variable angle prism is arranged to deflect the optical axis by varying its apex angle. Accordingly, the variable angle prism varies the apex angle according to a vibration applied to the photographic apparatus, thereby deflecting the optical axis so that the optical axis is made stable with respect to the image pickup device to effect stabilization of an incident image. Therefore, what is required for the mechanical driving method for varying the apex angle of the VAP is to incline the apex angle so that the optical axis is stably deflected in accordance with a control signal.
However, the above-described image-shake correcting device has a number of problems which will be described below.
FIGS. 2(a) and 2(b) show the frequency characteristics of the vibration component outputted from the HPF 2 when a vibration of constant amplitude is applied to a photographic apparatus including the image-shake correcting device of FIG. 1 which uses an existing type of angular-velocity sensor. FIG. 2(a) shows a gain characteristic, and FIG. 2(b) shows a phase characteristic.
Referring to the frequency characteristics of the vibration component of a vibration whose frequency is 10 Hz, the gain at 10 Hz is approximately 0 dB and no vibration component is detected, so that it may seem that a sufficient image stabilization effect is attained. However, the corresponding phase shown in FIG. 2(b) exhibits a deviation of approximately 7.5 degrees. Assuming that the frequency characteristics of an image correcting system (the VAP and the like) are ideal (i.e., a gain of 0 dB and no phase deviation over the entire image-shake correction frequency range), if an image stabilization effect, which is influenced by a phase deviation occurring in the vibration detecting system due to the phase deviation of 7.5 degrees, is calculated on the basis of Equation "20 log(OUT/IN)=G (gain)", it is understood that the aforesaid vibration is suppressed to 1/8.
In the above-described case, during normal photography, it is possible to achieve a sufficiently high, image stabilization effect. However, if a vibration of frequency in the neighborhood of 10 Hz is continuously applied to the photographic apparatus for a long time, the vibration may become steady to a visually perceptible extent.
In other words, if the photographic apparatus is exposed to a continuous vibration occurring in the frequency range in which no sufficient, vibration suppression effect can be achieved by the existing angular-velocity sensors, it is impossible to completely correct an image shake if the applied vibration reaches a certain magnitude.